This invention deals with high speed machining and more particularly with adaptations allowing to displace the tool in optimal conditions for classical machining operations, that is:
drilling,
spot facing,
tapping,
and boring.
In order to perform said operations, machine tools are classically used that, supporting a rotating element, ensure both the displacement of the tool with respect to the subject piece so as to bring it into the machining position and the machining motion of the tool so as to allow it to perform the machining operation.
In the case where the subject piece requires several machining operations and in order to optimize the performing rates of such a piece, a special machine is classically used that is constituted by several parallel and prepositioned broaches comprising a tool each, and that only ensures the machining displacement of the said broaches towards the subject piece in order to allow them to perform the machining operation. These special machines adopt a concept that is classically called xe2x80x9cmultibroach head conceptxe2x80x9d used on machines of the transfer type, very much used in the field of high production machining and especially for the machining of automotive pieces.
The machine tool designers, always in order to answer to the optimization criteria of the production rhythms of the subject pieces, were led to specialize their machines, which up to the present constituted the best solution to the production rapidity criterion. Thus, in order to avoid idle machine time and especially the time necessary for machining tool changes, machine tools devote their kinematics only to one machining motion and one multibroach head and are totally dedicated to one machining. This specialization that allows the suppression of all kinematics or mechanical module ensuring other motions or tool changes and thus the improvement of the performing velocity of said machines as well as the production turnout, resulted however in the fact that their user was unable to reconvert them to another machining operation, especially when the product, comprising the subject piece requiring one of the machining operations performed by said machine tool, was abandoned and is no longer present or necessary on the market for which it was intended.
As the lifetime of products and especially of those intended for cars diminishes every day, the implementation of a transfer machine totally dedicated to the machining of only one piece, the product that includes it being subject to a very quick disappearance, becomes very rapidly a very heavy investment.
The recent developments of high speed machining associated with the use of a logical structure of a positioning with parallel architecture allow to contemplate a machining by means of a monobroach head and thus monotool with times comparable to those of an average machining by means of a multibroach head.
The main feature of a parallel structure consists in the fact that the setting into action of the mobile element on an axis does not require the setting into action of kinematic acting elements and devices necessary for the setting into action of said mobile element on other axis, which allows obtaining the above mentioned machining times. Thus, the parallel structure adopts a kinematics of the type that ensures the positioning and displacement of a mobile element and includes at least a fixed element, at least a mobile element and at least three acting elements, each comprising a fixed part and a mobile part, the fixed part of each acting element being interdependent with the fixed element, the mobile part of each acting element being interdependent with the mobile element by means of a linkage element.
The applicant noticed that the implementation of a logical structure with parallel architecture in order to apply it concretely in the industry or in a machining chain, presents many difficulties that led machine tool designers to remain at the prototype stages or to use doubtful compromises in the required technological choices, compromises diminishing the capacities contemplated in theory in a machine tool with parallel structure.
One of the drawbacks of such a structure consists in the multiplication of joints, one of the consequences of which is that the broach maintained at the end of the positioning arms creates an important lever crank with respect to the basis of the positioning arms and which is difficult to set. The solution to this lever crank problem was, for the time being, to place the axis of the broach and thus of the tool vertically, so that the gravity effects on the machining head are taken into account by the acting elements of the positioning arms.
Another drawback of the machine tools with parallel structure consists in the fact that they were designed with a total flexibility, that is with a broach able to be oriented according to six degrees of freedom including the possibility to change tools. This multiplication of functions of a machine tool slows down its machining capacity by multiplying idle machine times.
An implementation example in the field of machining with parallel structure is described in document nxc2x0EP 0791438. This document describes a possibility to embody a machine tool, the tool holder""s slide of which would be in a horizontal position. This possibility constitutes one of the hereabove mentioned compromises, in that the motion along the slide axis is not obtained through the parallel structure but through an independent motorization associated with a slide-holder sheath that is itself linked to the parallel structure. Thus, in this implementation possibility, the parallel structure ensures the motion of the slide-holder sheath along a vertical plane perpendicular to the axis of the slide itself, and the slide-holder sheath itself ensures the translation motion of the slide along its axis perpendicular to the above mentioned plane. This particular configuration is due to the above mentioned difficulties of embodying the actionning and the putting into motion of a slide-holder associated with a parallel structure implementing a plurality of joints.
Having noticed these facts, the applicant led researches on a preferred embodiment of a parallel structure, that is of the type where each linkage element is flexibly mounted on one hand on the mobile part of the acting element and flexibly mounted on the other hand on the mobile element, and of the type where the whole is arranged so as to the inclination and orientation in space of the mobile element remain unchanged, whatever the motions of the mobile parts of the acting elements, in order to constitute a moldable parallelogram, the mobile element being constituted by a support-sheath of a tool-holder broach and the aforesaid parallel structure being arranged so that the axis of the broach is horizontal.
These researches led to a new and creative implementation of a parallel structure allowing to keep all the advantages of a parallel structure while adapting it in an optimal manner to a specific application of the machining field.
According to the main feature of the invention, the implementation device of a parallel structure is remarkable in that it comprises a balancing means that is constituted of a guiding module of said support-sheath along an horizontal axis associated with a support module constituted by a jack that, allowing the displacement of said guiding module along a vertical plane perpendicular to the axis of the broach and allowing the rectilinear motions of said support-sheath along the X, Y and Z axis, supports the assembly support-sheath, broach and tool, so that the weight of the latter would not be transferred on said parallel structure.
The motions of the parallel structure of the invention are voluntary limited so as to diminish the number of parameters to be handled and because of the choice made by the applicant who only wants to use this kinematic structure for some applications for which it is particularly intended.
This orientation is especially advantageous in that it allows the performance of the same works as those obtained by specialized machine tools making up the transfer machines, and therefore facilitate the integration of a machine tool adopting such a structure in a classical machining chain.
Such an orientation is new and runs counter to any other machine tool concepts adopting a parallel structure in that the machine tool adopting such a structure will be limited to straight machinings. This new and particularly advantageous orientation of the logical structure of the device according to the invention, that is ensuring the rectilinear motions of a mobile element with horizontal axis, requires special implementation means, in particular a balancing device.
Contrary to the document mentioned in the prior art, this device allows the parallel structure to ensure the rectilinear motions of the slide along the three axis of motion. Thus, the device according to the invention allows to take advantage of all the possibilities of a parallel structure in the framework of rectilinear motions.
Another object of the invention deals with the machine tool adopting a device corresponding to the invention. The fundamental concepts of the invention having just been presented in their most elementary form, other details and features will come forth more clearly when reading the following description giving, as a non limiting example and with respect to the attached drawings, an embodiment of a device implementing a logical structure with parallel architecture according to the invention.